It is often desirable to measure very small intervals of time with a great deal of accuracy. For example, mass spectroscopy requires accurate particle flight time measurement, in which a particle's velocity is determined by measuring its transit time across a known distance. Other examples of applications requiring accurate time measurement are radar, navigation, range finding, and ballistics.
Existing time interval measurement methods fall within two broad categories: time to amplitude conversion and time to digital conversion. The first method converts a time interval to a proportional pulse amplitude. The second method converts a time interval into a digital value. A problem with the first method is susceptibility to noise, and a problem with both methods is the need for special interface circuitry to provide a digital output that may be easily converted to a time measurement.
For example, one existing implementation of the time to digital conversion method uses a periodic system clock synchronized to the event of interest. The time interval is measured by counting oscillator pulses with a counter. For vernier resolution, the oscillator pulses are fed to a tapped delay line that divides the oscillator period. The delay line taps are used as addresses to a random access memory where a read, add one, and write routine is performed every oscillator period. In this manner, each time interval representing an event is represented by output from the counter and the random access memory. An additional conversion process must operate on this data to produce the measured time interval.
Pulse counting devices are also used for frequency measurement. In some applications, pulse to pulse measurements, as well as measurements of the number of pulses, are desired. In this connection, the ability to examine transient signals, such as a single pulse, is useful.
A need exists for a time or frequency measurement device that simplifies circuitry and thereby minimizes weight and size. In this regard, the mass and size of prior devices have limited their usefulness for space borne applications. Additionally, the device should not compromise accuracy and speed, and should permit the propagation of a single pulse to be examined.